The underside of the piston was bright and shiny, no varnish or discoloration.  This thing runs cool.

And the rod swung to the right +.006” for a total of .012”.

Looked like the rod was fine.  Not a whole lot more to choose from.  Gotta be one of the ball/roller bearings or the transmission.  When I rotated the input and output shafts, I could find spots where it would bind.  It took a significant amount of force to rotate the shafts by hand.  You can see all the input gears through the opening in the crankcase.  Might as well look.

Sorry, my pictures aren’t the best.  More to come once I get it opened up.  I’m dying to see what the output gear looks like.

Here’s another shot of the input gear.  You can really get a feel for the depth of the pits from this shot.  The left side of each tooth is really cratered.

Does this mean we should be wary of the 97mm flat-top piston mod?  I don’t think so.  You must consider what I have been doing to this thing.  The bike has a little over 25,000 miles on it.  Of those 25,000 miles, about 1,500 were logged with the spare engine.  The remaining 23,500 miles were logged on the current bottom end & transmission.  Most of those 23,500 miles were logged beating the crap out of it.  All sorts of crazy engine combinations.  Hundreds of WOT pulls.  A whole bunch of LSR runs.  Countless hours spent cruising at 4500 to 5000 rpm.  Repeated trips to 7500 rpm, over & over & over & over.  This baby is my test lab.  Its sole purpose is to develop and test modifications.  Failure was inevitable.

Fast 650 thought it would be the engine case (I fooled him).  I personally thought it would be the rod, or an exhaust valve.  “IF” I had been running a magnetic drain plug, I might have caught the problem in the early stages of failure.  Maybe I would not have ruined the cylinder.  Lesson learned.

Once I get it all apart, I will put the crank between centers and do a proper runout check.  I’m hoping it is not tweaked, but if it is, it will be a good opportunity to learn about truing a crankshaft.

I’m guessing fifth gear has been goin in the potty for a long time, probably started when I did the first tight-quench mod in 2019.  That bad-boy failed at WOT in fifth gear.  Insufficient ring gap.  Pretty ugly.  Maybe the pitting in the gear teeth started with that setup.  It had a lot of compression and made about 42 HP.  I rinsed out the bottom end with clean oil, installed a new cylinder with a Wiseco pop-top, and forged ahead.

The fretting on the output splines is superficial.  It cleaned up nicely.  I’ve been reading up on spline fretting and all the literature seems to point to lubrication.  So, I’m gonna start painting my splines with moly lube when I install the pulley.  Not the threads, just the splines.

I’ve got several things I want to test, so to keep the program going I installed the spare engine.  It’s set up with a 94mm flat-top, DR650 cam, and Stage II cylinder head.  It runs great.  The clutch is fine, no slippage, good lever action.  To smooth out power delivery, I installed the stock flywheel.  Any WOT work above 5000 rpm will be reserved for the lower four gears.  I’ll be using fifth gear strictly for cruising.

I plan to rebuild the 97 mm flat-top using a four-speed transmission.  Since I have two 97mm pop-top pistons, I want to see if I can turn one of those into a flat-top piston and adjust the cylinder height to achieve reasonable quench and about 10.5:1 compression.  That looks like it will be a lot of fun.  The transmission conversion will certainly be interesting.
This concludes the project.  

I hope some of you found my reports informative and can use the data I collected to help make decisions on your own project.  I think the final configuration with the 97 mm flat-top piston is a good setup as long as you exercise prudent use of the throttle in fifth gear.  It’s a blast to ride.

Best regards, Mike    

An interesting report, thank you Mike.

4/17/22  Teardown Inspection Results

Now I have the engine completely disassembled and have finished inspecting everything.  I wanted to share what I found and offer my assessment.  I’m not an engineer or a metallurgist, and I don’t have a materials lab, so the comments are simply my opinion.

Before splitting the case, I used a magnet to fish around in the pool of oil directly below the input gears.  There were plenty of metal flakes in there.

5/7/22:  Boy did I mess this up.  Sorry, I forgot to paste in the text associated with the picture.  Never too late to patch it up.  This text was supposed to be in the original reply.

I got the cases split and immediately threw the crank shaft between centers.  I was worried that it might be tweaked.  It looks great, but centers don’t lie.

My initial assessment was correct.  It’s running out .004” TIR with the plus (+) directly opposite the crank pin.  That confirms that the webs are spread.    Fortunately, it’s not twisted.  I might be able to straighten it.  Time to learn something new.

The rollers look a bit scored but overall seem to be in decent shape.

8/25/23  The inner race had failed, but you can't see the inner race unless you cut the bearing apart.